Radix UI unstyled accessible React components. Headless primitives. USE WHEN: user mentions "Radix", "Radix UI", "headless components", "unstyled primitives", asks about "accessible components", "Radix primitives" DO NOT USE FOR: shadcn/ui (use shadcn-ui skill), styled component libraries (Material-UI, Chakra), Vue/Svelte (use Headless UI)

REST Assured Java library for testing REST APIs. Covers HTTP requests, JSON/XML validation, and Spring Boot integration. USE WHEN: user mentions "rest assured", "api test java", "rest api test", asks about "given when then", "RestAssured", "HTTP test", "JSON validation" DO NOT USE FOR: Unit tests - use `junit`; E2E browser tests - use Selenium; WebSocket testing - use dedicated tools; Non-Java projects - use language-specific HTTP clients

Socket.IO real-time bidirectional communication. Rooms, namespaces, acknowledgments, middleware, scaling with Redis adapter, and TypeScript types. USE WHEN: user mentions "Socket.IO", "socket.io", "real-time chat", "live updates", "bidirectional WebSocket", "rooms", "namespaces" DO NOT USE FOR: SSE (server-sent events) - use `sse`; WebRTC - use `webrtc`; raw WebSocket without Socket.IO - use framework WS skills

Swashbuckle and NSwag for ASP.NET Core API documentation. Covers XML comments, operation filters, and OpenAPI customization. USE WHEN: user mentions "Swagger", "Swashbuckle", "NSwag", ".NET OpenAPI", "API documentation", "Swagger UI", ".NET API docs" DO NOT USE FOR: Springdoc OpenAPI - use `springdoc-openapi`, generic OpenAPI spec - use `openapi`

Terraform infrastructure as code. HCL syntax, providers (AWS, Azure, GCP), modules, state management, workspaces, and production patterns. USE WHEN: user mentions "Terraform", "HCL", "infrastructure as code", "IaC", "terraform plan", "terraform apply", "tfstate", "OpenTofu" DO NOT USE FOR: Kubernetes manifests - use `kubernetes`; Docker - use `docker`; Pulumi - different IaC tool

typescript-eslint - ESLint plugin for TypeScript USE WHEN: user mentions "typescript-eslint", "TypeScript linting", "type-aware rules", asks about "no-floating-promises", "TypeScript ESLint config", "@typescript-eslint rules" DO NOT USE FOR: ESLint 9 full setup - use `eslint-biome` skill, Biome - use `eslint-biome`, general quality - use `quality-common`

Unity Addressables — AssetReference, async loading, labels, groups, build profiles, remote content, Cloud Content Delivery integration, content catalogs and updates. USE WHEN: replacing `Resources.Load`, async asset loading, scene streaming, remote content, DLC delivery, runtime asset hot-swap, splitting builds for size. DO NOT USE FOR: editor-time asset queries (use AssetDatabase); legacy Asset Bundles workflows (Addressables wraps them).

Unity Test Framework — EditMode + PlayMode tests, [UnityTest] coroutines, Test Runner, mocking patterns, Unity.PerformanceTesting package. USE WHEN: writing/debugging tests, setting up CI test runs, performance regression tests, asserting scene state, mocking time/input. DO NOT USE FOR: runtime in-game telemetry (use `unity-performance` for profiling); end-to-end automation (use editor tooling + custom harness).

WebRTC peer-to-peer communication. Signaling, ICE/STUN/TURN, media streams, data channels, screen sharing, and SFU integration (mediasoup, LiveKit). USE WHEN: user mentions "WebRTC", "video call", "peer-to-peer", "P2P", "screen sharing", "data channel", "STUN", "TURN", "mediasoup", "LiveKit" DO NOT USE FOR: server-to-client streaming - use `sse`; chat messaging - use `socket-io`

Blazor Server, WebAssembly, and United (Auto) with components, interop, and render modes. Covers .NET 8+ Blazor patterns. USE WHEN: user mentions "Blazor", "Blazor Server", "Blazor WASM", "Blazor WebAssembly", "Blazor components", "render modes", "Blazor interop" DO NOT USE FOR: Angular components - use `angular`, React components - use `react`, Svelte - use `svelte`

Bitcoin Improvement Proposals index, organized by topic. Active soft forks, wallet standards, network protocol, format standards. Pointers to canonical text and historical context. USE WHEN: looking up a BIP number, citing a standard, or understanding the relationship between related BIPs.

Bitcoin consensus rules: block validation, transaction validation, soft-fork activation mechanics (BIP9, BIP8, Speedy Trial), nLockTime / nSequence semantics, mempool policy vs consensus distinction. USE WHEN: discussing what makes a tx/block valid, soft-fork deployment, reorg handling, "is this consensus-valid".

Bitcoin Core descriptors wallet: createwallet, importdescriptors, multipath, ranges, watch-only, signing wallets, multisig coordination. Replaces legacy BDB wallet. USE WHEN: setting up Bitcoin Core wallet, watching xpubs, building signing pipelines via Core.

Bitcoin Core release engineering: Guix reproducible builds, signed release tarballs, deterministic outputs, code-signing keys, the release process, security disclosure. USE WHEN: building Bitcoin Core from source for verification, understanding release security, contributing to consensus-critical code.

Bitcoin Core REST interface: read-only HTTP endpoints for blocks, txs, UTXOs, headers, mempool. No auth, no rate limit. Useful for high-throughput read services. USE WHEN: building read-only services that don't need RPC auth, fetching tx/block data efficiently, integrating with explorers.

ZeroMQ notifications from Bitcoin Core: rawblock, rawtx, hashblock, hashtx, sequence. Subscribe over TCP/IPC, integration patterns for Electrs, BTCPay, Lightning nodes. USE WHEN: building real-time integrations with bitcoind, monitoring mempool sequence, integrating LN nodes.

Fee estimation strategies: local mempool-based (estimatesmartfee), external APIs (mempool.space, BlockCypher), bitcoind's smart-fee algorithm, replacement bidding, fee-rate floor (incremental relay). USE WHEN: choosing a fee for a new tx, evaluating fee-bump amounts, designing fee policy for a service.

Multi-vendor multisig: combining HW wallets from different vendors (Trezor + Ledger + Coldcard) for diversification. Coordinator-mediated setup, descriptor sharing, threat model. USE WHEN: designing multi-vendor custody, implementing 2-of-3 multisig with diverse hardware.

Bitcoin Knots: Luke Dashjr's Bitcoin Core fork with stricter mempool policy and additional configuration. Differences vs Core, when to consider, compatibility. USE WHEN: evaluating Knots for stricter mempool policy, reviewing a node deployment that uses Knots, understanding policy divergence between Core and Knots.

Ark protocol: VTXOs (Virtual Transaction Outputs) batched off-chain with periodic on-chain settlement. ARKADE (Ark Labs) is the first mainnet implementation. ASP (Ark Service Provider) coordinates rounds; users keep unilateral exit. USE WHEN: building Ark integrations, evaluating Ark vs Lightning, designing batched payment flows.

Taproot Assets (formerly Taro) by Lightning Labs: client-side validation asset protocol on Bitcoin + Lightning. Multi-asset Lightning channels, AddressV2, multi-hop asset routing. USE WHEN: issuing/transferring assets on Bitcoin, integrating asset Lightning channels, evaluating TAP vs RGB.

BIP47 reusable payment codes (PayNyms): notification transactions, derived per-counterparty addresses, Diffie-Hellman shared-secret derivation. Privacy-preserving alternative to address rotation. USE WHEN: implementing PayNyms / Sparrow / Samourai-compatible payment codes, evaluating BIP47 vs Silent Payments.

Replace-by-Fee (BIP125, full-RBF) and Child-Pays-For-Parent fee bumping. Mempool ancestor/descendant rules, package CPFP (BIP331), TRUC v3 (BIP431). Wallet-level fee bump strategies. USE WHEN: implementing fee bumping, debugging "stuck transaction", designing wallet UX for fee acceleration.

Segregated Witness: BIP141 (consensus), BIP143 (sighash), BIP144 (P2P serialization), BIP173 (Bech32 address). Witness segregation, weight units, txid vs wtxid, witness commitment. USE WHEN: working with native segwit (P2WPKH/P2WSH), embedded segwit (P2SH-P2WPKH/P2SH-P2WSH), or understanding the witness discount.

Bitcoin vault patterns: timelocked cooldown, hot/cold path with CSV, multisig vaults with quorum reduction over time, OP_VAULT proposal (BIP345), Revault architecture. USE WHEN: designing high-value custody, treasury setups, recovery schemes with publish-and-cooldown semantics.

Wallet backup formats: BIP39 mnemonic, SLIP-39 Shamir Backup, SeedQR, CompactSeedQR, Coldcard's CTL-format, paper backup hygiene, metal backup vendors and threat models. USE WHEN: designing backup UX, evaluating backup security, helping recover a backed-up seed.

BIP85 deterministic entropy: derive child entropy/seeds from a parent BIP32 path. Used for sub-wallets, cross-app keys, deterministic randomness without separate backups. USE WHEN: spawning sub-wallets from one master seed, generating encryption keys deterministically, building a "memory" of one seed.

ESLint - pluggable JavaScript/TypeScript linter USE WHEN: user mentions "ESLint", "JavaScript linting", ".eslintrc", asks about "eslint config", "linting rules", "eslint plugins", "flat config" DO NOT USE FOR: Biome - use `eslint-biome` skill, TypeScript-specific - use `typescript-eslint` skill, code quality principles - use `quality-common`

Flutter cross-platform development with Dart. Widgets, state management (Riverpod, BLoC), navigation, platform channels, and production patterns. USE WHEN: user mentions "Flutter", "Dart", "flutter widget", "Riverpod", "BLoC", "flutter build", "pubspec.yaml" DO NOT USE FOR: React Native - use `react-native`; Expo - use `expo`; native iOS/Android

IEC 61131-3 programming languages reference (LD, FBD, ST, IL, SFC), PLCopen standards, program organization units, data types. Use when discussing PLC programming standards or cross-platform compatibility.

Multi-Path Payments (MPP) — split a payment across multiple parallel routes; same payment hash, payment_secret binds parts. AMP (Atomic Multipath) — variant where each part has different hash; receiver derives shared secret. USE WHEN: implementing payment splitting, debugging "couldn't route full amount", evaluating MPP vs AMP.

BOLT 7 gossip: node_announcement, channel_announcement, channel_update. Sync via gossip_queries, channel range queries, gossip rate limiting, pruning stale announcements. USE WHEN: implementing/debugging gossip sync, managing graph storage, designing rate limits.

Greenlight by Blockstream: hosted CLN with self-custodial signing. Server runs the full CLN node; user retains private keys via signer protocol. Mobile-friendly without running a full LN node. USE WHEN: building mobile/web LN apps with self-custody but no on-device daemon, integrating with Blockstream's hosted infra.

LND (Lightning Network Daemon) by Lightning Labs: gRPC + REST API, macaroons, lncli, autopilot, channel backups (SCB), Tor support, watchtowers, integration with bitcoind/btcd/Neutrino backend. USE WHEN: integrating with LND, scripting lncli, debugging LND-specific issues, designing LND-based services.

LSP (Lightning Service Provider) specifications: BLIPs, JIT channels, on-pay channel open, MPP-trampoline, BOLT12 hosted invoice. Major LSPs: ACINQ (Phoenix backend), Voltage, Olympus, Megalith, Flashsats. USE WHEN: integrating with an LSP, designing LSP services, evaluating LSP options for a wallet.

Sphinx onion routing for Lightning (BOLT 4): packet structure, per-hop encryption, payload TLV, blinded paths, onion messages. USE WHEN: implementing onion construction/decryption, debugging routing payloads, designing onion-based off-chain protocols.

Replacement cycling attack on Lightning (Riard 2023): exploits BIP125 rule 5 to indefinitely delay an honest HTLC-timeout, costing the victim. Mitigated by TRUC v3 + ephemeral anchors. USE WHEN: auditing LN security posture, evaluating fee-bumping strategies, understanding why TRUC was activated.

Channel splicing: splice-in (adding capacity) and splice-out (withdrawing) without closing the channel. Replaces "close and reopen" with one splice tx. USE WHEN: increasing/decreasing channel capacity in production, designing dynamic-balance LSP integrations, planning channel lifecycle.

Watchtower architecture: encrypted breach blob storage, justice transactions, monitoring outsourced for offline channels. Eltoo proposal alternatives, BOLT 13 (proposed). USE WHEN: deploying a watchtower service, evaluating watchtower vendors, designing self-hosted monitoring.

WebLN: browser API for Lightning. window.webln exposed by wallet extensions / mobile webviews. Methods: enable, getInfo, sendPayment, makeInvoice, signMessage. Bitcoin Connect (modern wrapper). USE WHEN: building web apps with Lightning payments, integrating with Alby/getalby, designing WebLN-compatible flows.

NUnit testing framework with TestCase, SetUp/TearDown, assertions, and parameterized tests for .NET applications. USE WHEN: user mentions "NUnit", "TestCase", "TestFixture", "NUnit assertions", "NUnit parameterized tests", ".NET NUnit" DO NOT USE FOR: xUnit - use `xunit`, MSTest, JavaScript testing frameworks

SQL Server and Azure SQL Database with T-SQL, indexes, stored procedures, and performance optimization patterns. USE WHEN: user mentions "SQL Server", "T-SQL", "MSSQL", "Azure SQL", "SQL Server indexes", "stored procedures", "SQL Server performance" DO NOT USE FOR: PostgreSQL - use `postgresql`, MySQL - use `mysql`, MongoDB - use `mongodb`

Bitcoin Core indexes: txindex, blockfilterindex (BIP157/158), coinstatsindex. Disk costs, rebuild times, when each is needed. USE WHEN: choosing which indexes to enable, troubleshooting "txid not found", integrating with electrs / fulcrum / LN nodes.

ECDSA signatures over secp256k1: sign/verify, deterministic-k (RFC6979), low-s (BIP146), DER encoding, malleability, recoverable signatures. USE WHEN: signing legacy/SegWit-v0 inputs, verifying legacy signatures, debugging "non-canonical signature" errors.

Esplora: Blockstream's HTTP-based block explorer + indexer. REST API at /api/<endpoint>. Powers blockstream.info, mempool.space's earlier versions, and self-hosted explorers. USE WHEN: deploying a personal block explorer, integrating with Esplora REST API, building wallet / monitoring on Esplora-style endpoint.

Babylon: native BTC staking protocol securing PoS chains. Trustless Bitcoin Vaults via BitVM3, integration with Cosmos chains. No bridging / wrapping required. USE WHEN: building BTC-secured PoS chains, integrating Babylon staking, evaluating BTCFi yield options.

BitVM, BitVM2, BitVM3: off-chain computation framework using optimistic challenge games on Bitcoin script. Foundation for trust-minimized bridges to L2s and PoS chains. USE WHEN: building trust-minimized bridges, understanding ZK rollup peg-out mechanisms, evaluating bridge security models.

Cashu: single-mint Chaumian e-cash for Bitcoin Lightning. NUTs (Notation, Usage, and Terminology specs) define mint protocol, payment flow, blind signatures. USE WHEN: integrating Cashu mints/wallets, building NUT-compliant apps, comparing with Fedimint.

Drivechains (BIP300/301) and Spacechains (Ruben Somsen): proposals for sidechains pegged via miner-validated locks or via OP_CAT-like primitives. Status: NOT activated on mainnet. USE WHEN: evaluating proposed sidechain mechanisms, understanding why they're not live, comparing with federated/trustless bridges.

Fedimint: federated Chaumian e-cash on Bitcoin with Lightning gateway. Multi-sig guardians, blind-signature notes, community-scale custody with privacy. Fedi mobile wallet. USE WHEN: deploying community mints, integrating federated e-cash, evaluating Fedimint vs Cashu.

Liquid Network: federated sidechain by Blockstream. Confidential Transactions, asset issuance (LBTC, USDt-Liquid, others), 2-min blocks, n-of-m federation peg-out. Elements codebase. USE WHEN: building on Liquid, peg-in/peg-out integrations, designing CT-based privacy.

RGB protocol: client-side validated smart contracts on Bitcoin + Lightning. Schema-driven contracts, AluVM (RGB VM), rgb-lightning-node for LN integration. USDT-on-RGB rollout. USE WHEN: building RGB contracts, integrating RGB Lightning, comparing with TAP.

Spark by Lightspark: statechain-derived L2 with FROST threshold signatures, leaf architecture, Lightning compatibility. Beta launched April 2025; managed by Spark Operators (SOs). USE WHEN: integrating with Spark, evaluating Spark vs Ark vs Lightning, designing leaf-based UTXO trees.

Stacks: Bitcoin smart contract layer with Clarity language, sBTC for native peg, Nakamoto upgrade for fast finality, PoX (Proof of Transfer) consensus. USE WHEN: building Bitcoin smart contracts, integrating sBTC, evaluating Stacks vs RSK / EVM alternatives.

Bitcoin Dev Kit (BDK): high-level Rust wallet library. Descriptor-based wallets, multiple chain backends (Esplora, Electrum, RPC), PSBT, fee estimation, coin selection. Bindings to Swift / Kotlin / Python / Flutter. USE WHEN: building wallets in Rust or via BDK bindings, integrating descriptor wallets into apps.

BRC-20: fungible token standard via JSON inscriptions. Deploy / mint / transfer operations. Permissionless and unaudited; large ecosystem post-2023. USE WHEN: building BRC-20 tooling, evaluating token issuance on Bitcoin, comparing with Runes / Taproot Assets.

Inscriptions: arbitrary data committed to specific sats via Taproot envelope structure. Foundation for BRC-20, recursive inscriptions, "Bitcoin NFTs". USE WHEN: creating/parsing inscriptions, debugging inscription flow, evaluating inscription cost / fee impact.

Stratum V1: TCP/JSON protocol between mining pool and miners. Subscribe, authorize, mining.notify, mining.submit. Most-used protocol but security/decentralization weaknesses. USE WHEN: integrating mining hardware, building pool / proxy software, debugging hash submission.

Stratum V2: encrypted, miner-driven mining protocol. Job Declaration, Template Provider, allows miners to choose their own tx set. Defeats pool-level censorship. USE WHEN: deploying SV2-capable mining infrastructure, evaluating V1 vs V2, integrating with Braiins Pool / Datum.

Cross-chain atomic swaps: BTC ↔ LTC / BTC ↔ Liquid / BTC ↔ Monero via HTLC or adaptor signature schemes. Trustless swaps without centralized exchange. USE WHEN: building atomic swap services, evaluating cross-chain bridges, integrating with COMIT / submarine-swap variants.

CoinJoin: collaborative tx where multiple users combine inputs to break common-input-ownership heuristic. Wabisabi (Wasabi 2.0), Whirlpool (Samourai), JoinMarket maker/taker. USE WHEN: implementing CoinJoin coordinator/client, evaluating privacy gain, designing post-CoinJoin hygiene.

PayJoin (BIP78 + v2): receiver-augmented payment that breaks common-input-ownership heuristic. v2 adds async relay support. USE WHEN: implementing PayJoin sender/receiver, designing privacy- preserving payment flows, integrating with payjoin servers.

Silent Payments (BIP352): single static address from which sender derives unique output per payment. Schnorr-based blinding; no notification tx needed. USE WHEN: implementing BIP352 client/wallet, evaluating vs BIP47, understanding scan cost trade-offs.

secp256k1 elliptic curve: parameters, group law, key encoding, point serialization, library bindings (libsecp256k1 in C, Rust, Python). USE WHEN: low-level key/point operations, debugging signature failures, understanding why Bitcoin uses this curve specifically.

Bitcoin time-locks: nLockTime (BIP65 absolute), nSequence (BIP68/112 relative), CLTV/CSV opcodes, MTP rule (BIP113). Block-height vs unix-time, encoding rules. USE WHEN: building HTLCs, vaults, scheduled payments, escrow with cooldown, refund paths.

Bitcoin payment URI scheme (BIP21), Lightning URIs, BIP70/72 (mostly deprecated), PayJoin URI extension (BIP78), unified URIs combining on-chain + Lightning. USE WHEN: parsing/generating bitcoin: URIs, building QR codes, designing wallet payment requests.

BIP329 wallet labels portability: standard JSON Lines format for exporting/importing labels (addresses, txs, UTXOs, xpubs, inputs, outputs) across wallet software. USE WHEN: importing/exporting label sets, integrating with multiple wallets that should share annotations.

# Kotlin Security Skill

BOLT12 offers: reusable invoices, blinded paths, recurring payments, multi-asset (Taproot Assets layered). Replaces BOLT11 for many use cases. USE WHEN: implementing/integrating offers, designing recurring payments, evaluating BOLT11 vs BOLT12 trade-offs.

Channel jamming attacks: slow-jam (long-held HTLCs exhaust slots), fast-jam (rapid hold/release exhausts liquidity). Mitigations: upfront fees, reputation, hold-fee proposals. USE WHEN: assessing LN security model, designing reputation systems, evaluating jamming-resistant routing.

Eclair Lightning implementation by ACINQ: Scala/Akka, eclair-cli, plugin system, primarily backend service. Powers Phoenix wallet's server side and ACINQ's main routing node. USE WHEN: deploying Eclair as backend, integrating with ACINQ's Phoenix backend, evaluating Scala/JVM Lightning options.

Keysend / spontaneous payments: send Lightning to a destination without an invoice. Receiver derives preimage from sender-supplied TLV record. BOLT 9 bit 50. USE WHEN: implementing tipping flows, designing zero-friction micro-payments, evaluating keysend vs BOLT11 invoice flow.

Lightning Labs liquidity tools: Loop (off-chain ↔ on-chain swaps), Pool (auction-based liquidity market), Lightning Terminal (Lit, web UI). Submarine swap mechanics behind Loop. USE WHEN: managing channel liquidity, evaluating Loop vs Boltz vs Peerswap, deploying Lit for fleet management.

Nostr Wallet Connect (NWC, NIP-47): wallet remote control via Nostr relays. App on one device drives wallet on another via signed JSON messages over Nostr. USE WHEN: integrating wallet-side or app-side NWC, designing remote-control flows, evaluating NWC vs WebLN.

Pinning attacks: low-fee descendant txs block honest fee-bumping by exploiting mempool descendant policy. Anchor outputs (BOLT-3), TRUC v3 (BIP431), and ephemeral anchors as defenses. USE WHEN: auditing fee-bump robustness, evaluating channel security, reviewing post-2024 LN deployments.

Submarine swaps: atomic on-chain ↔ Lightning conversions via shared HTLC preimage. Boltz protocol, Loop protocol, Peerswap (P2P), trustless variants with adaptor signatures. USE WHEN: building swap services, integrating with Boltz/Loop, designing trust-minimized swap flows.

Simple Taproot Channels (BOLT 9 bit 56): MuSig2-aggregated funding output, Tapscript commitment leaves, taproot-native HTLCs. USE WHEN: enabling taproot channels in your impl, evaluating cooperative-spend privacy gains, designing taproot HTLC scripts.

UMA (Universal Money Address): Lightspark's spec extending Lightning Address with non-USD currencies, KYC compliance, sender attestation, multi-currency exchange via Lightning routing. USE WHEN: integrating with UMA-enabled providers (mostly enterprise), building cross-currency LN payment flows.

# API Security Skill

Coldcard hardware wallets (Coinkite): Mk4, Q. Bitcoin-only firmware, airgap support via SD card / NFC / QR, BIP85, MuSig2 (experimental), Trick PINs. USE WHEN: integrating with Coldcard, supporting Mk4 / Q, designing airgap signing flows.

HWI (Hardware Wallet Interface): standardized Python API across HW vendors (Trezor, Ledger, Coldcard, BitBox02, Jade, etc.). Used by Bitcoin Core, Sparrow, Specter for cross-vendor signing. USE WHEN: integrating multi-vendor HW signing, building wallets that support several HW.

Ledger hardware wallets: Nano S Plus, Nano X, Stax, Flex. BOLOS app architecture, Ledger Live, secure element, anti-klepto. USE WHEN: integrating with Ledger, supporting Ledger in app, debugging PSBT signing or HW Recovery service issues.

PSBT signing flow patterns across HW vendors: USB direct, microSD airgap, QR airgap, NFC, multi-device coordination. USE WHEN: designing HW signing UX, debugging cross-device PSBT flow, supporting multi-vendor multisig.

Trezor hardware wallets: Model T, Model One, Trezor Safe 3, Safe 5. Trezor Suite, trezorctl, USB protocol, BIP32 derivation, passphrase support, Shamir Backup. USE WHEN: integrating with Trezor, supporting Trezor in app, troubleshooting Trezor signing.

BTCPay Server: open-source merchant payment processor. Self-hosted, supports BTC + Lightning + altcoins, plugin ecosystem, Lightning Address resolver, point-of-sale. USE WHEN: deploying merchant payments, integrating with BTCPay, building plugins.

mempool.space: open-source mempool explorer + accelerator. Detailed fee tracking, mempool blocks projection, REST + WebSocket APIs. Self-hostable. USE WHEN: integrating mempool.space API, deploying self-hosted variant, evaluating fee estimation services.

Self-hosted Bitcoin node distros: Umbrel, Start9 Embassy, RaspiBlitz, MyNode, Citadel. Pre-configured Bitcoin + Lightning + apps. USE WHEN: deploying personal node infrastructure, choosing between distros.

Sparrow Wallet: power-user desktop wallet for Bitcoin. PSBT-first, HW wallet support, multisig coordination, CoinJoin (Whirlpool fork), PayJoin, BIP47. USE WHEN: power-user wallet workflows, evaluating Sparrow vs Specter / Electrum.

Citrea: first ZK-rollup on Bitcoin (mainnet 27 Jan 2026), zkEVM, BitVM2 trustless bridge, ctUSD stablecoin, native BTC lending. USE WHEN: building zkEVM apps on Bitcoin, integrating with Citrea, evaluating ZK rollups vs sidechains.

Counterparty: legacy Bitcoin meta-protocol (2014) for assets, DEX, betting using OP_RETURN. Largely historic; still alive with niche use. USE WHEN: dealing with legacy Counterparty assets, understanding pre-Ordinals asset ecosystem, evaluating obsolete patterns.

Rootstock (RSK): EVM-compatible Bitcoin sidechain with merge mining and federation peg. RBTC native token, 30-second blocks, Solidity smart contracts. USE WHEN: building Bitcoin DeFi via EVM contracts, integrating RSK peg, comparing with Stacks / Liquid.

Statechains: off-chain UTXO ownership transfer via Statechain Operator (SO) signing. Mercury Layer (CommerceBlock) is the main implementation. 1-of-N security model. USE WHEN: evaluating off-chain UTXO transfer, comparing with Spark/ Ark, designing 1-of-N security models.

rust-miniscript: Rust crate for Miniscript and descriptor handling. Policy compilation, satisfaction analysis, descriptor parsing for legacy + Tapscript. USE WHEN: working with descriptors / miniscript in Rust, building wallets that compose spending conditions.

rust-bitcoin: Rust crate for Bitcoin primitives. Tx, Block, Script, Address, Network, key types, PSBT, BIP32. Foundation for BDK, LDK, miniscript-rs. USE WHEN: building Bitcoin apps in Rust, parsing/constructing txs, working with addresses + scripts.

Ordinal theory: numbering scheme for individual satoshis (sat ordinals). ord client, sat-tracking, rare sat collecting, foundation for inscriptions / BRC-20 / sub-protocols. USE WHEN: working with ord client, tracking sats, building Ordinals apps.

Runes: Casey Rodarmor's UTXO-native fungible token protocol. Activated at halving April 2024. Smaller on-chain footprint than BRC-20, native validation rules. USE WHEN: building Runes apps, comparing with BRC-20 / Atomicals.